Most Common Bacteria Causing Complicated UTIs in Adults
In adults with complicated UTIs, Escherichia coli remains the most common pathogen, but the microbial spectrum is significantly broader than in uncomplicated infections, with Klebsiella spp., Proteus spp., Pseudomonas spp., Serratia spp., and Enterococcus spp. representing the other major causative organisms. 1
Primary Bacterial Pathogens in Complicated UTIs
The microbial landscape differs substantially from uncomplicated UTIs:
E. coli is the single most common organism, though it occurs proportionally less frequently than in uncomplicated UTIs 1, 2, 3
The following organisms constitute the expanded spectrum in complicated infections: 1
- Klebsiella pneumoniae and other Klebsiella species
- Proteus species (P. mirabilis, P. vulgaris)
- Pseudomonas species (including P. aeruginosa)
- Serratia species (S. marcescens)
- Enterococcus species
Additional pathogens frequently isolated include: 4, 5
- Staphylococcus aureus (both penicillinase and non-penicillinase producing)
- Staphylococcus epidermidis
- Enterobacter species
- Morganella morganii
- Anaerobic organisms (Bacteroides fragilis, Clostridium species, Peptostreptococcus species) in specific complicated scenarios
Key Distinguishing Features from Uncomplicated UTIs
Antimicrobial resistance is substantially more likely in complicated UTIs compared to uncomplicated infections. 1
- E. coli strains isolated from complicated UTIs demonstrate fewer virulence characteristics but higher resistance rates than those from uncomplicated infections 1
- Extended-spectrum β-lactamase (ESBL)-producing organisms are a particular concern in complicated UTIs 1
- Multidrug-resistant uropathogens represent an increasingly important consideration in management 1
Clinical Context for Bacterial Spectrum
The broader microbial spectrum reflects the underlying host and anatomic factors:
- Patients with structural/functional urinary tract abnormalities harbor more diverse organisms 1
- Healthcare-associated infections increase the likelihood of resistant organisms 1
- Catheter-associated UTIs approach 100% prevalence of bacteriuria with long-term catheterization, involving polymicrobial flora 1
- Male patients with UTI (nearly always considered complicated) show higher rates of coagulase-negative staphylococci, gram-negative bacilli, and Enterococcus species 1
Practical Implications for Empiric Therapy
Always obtain urine culture and susceptibility testing before initiating therapy in complicated UTIs, as the expanded microbial spectrum and resistance patterns necessitate tailored antimicrobial selection. 1